THIS is the best brownie frosting EVER, and it comes from a mix:
Pamela's Dark Chocolate Frosting
The package looks like this, and if you need it faster than the mail you can buy it at Whole Foods.
Pamela's Products Dark Chocolate Frosting Mix, 12-Ounce Bags (Pack of 6): Amazon.com: Grocery
All natural. It isn't really dark chocolate, so I have no idea about the name. Two minutes to make. You will think you died and went to heaven.
I can't speak for anyone else on this thread, but trust me. I know brownies. I know frosting. I am a dessert goddess.